The present invention relates to light transmissive phase change inks printed onto substrates and to methods for producing such printed substrates.
In general, phase change inks are in solid phase at ambient temperature, but are liquid at the elevated operating temperature of an ink jet printing device. Liquid phase ink jet drops at the operating temperature are ejected from the printing device and, when the ink drops contact the surface of printing media, they quickly solidify to form a predetermined pattern.
Phase change ink is desirable since it remains in a solid phase at room temperature during shipping, long-term storage, etc. Also, the problems associated with nozzle clogging due to ink evaporation are largely eliminated, thereby improving the reliability of ink jet printing. Furthermore, since the ink droplets rapidly solidify upon contact with the substrate, migration of ink along the printing medium is greatly reduced and image quality is improved. Rapid solidification allows high quality images to be printed on a wide variety of printing media.
The initial prior art on phase change inks for ink jet printing involved monochrome inks jetted by electrostatic printing devices. Thus, for example in U.S. Pat. No. 3,653,932, a low melting ink (30.degree. C. to 50.degree. C.) is provided employing an ink base comprising di-esters of sebacic acid. In a similar process, U.S. Pat. No. 3,715,219 describes another low melting point ink (30.degree. C. to 60.degree. C.) comprising a paraffin alcohol-based ink. However, when low melting point phase change inks are employed in printing onto a substrate, they exhibit offset problems, namely, when the substrates printed with these inks are stacked and stored for subsequent use, they can become adhered to one another, particularly if high ambient temperatures are experienced.
U.S. Pat. Nos. 4,390,369 and 4,484,948 describe methods for producing monochrome phase change inks which employ a natural wax ink base, such as Japan wax, candelilla wax, carnauba wax, etc., which is printed from a drop-on-demand ink jet device at a temperature ranging between 65.degree. C. and 75.degree. C. In U.S. Pat. No. 4,659,383, a monochrome ink composition is provided having an ink base comprising a C20-24 acid or alcohol, a ketone, and an acrylic resin plasticizer. These monochrome ink compositions are not durable and when printed can be smudged with routine handling and folding.
In Japanese patent application No. 128,05,78, aliphatic and aromatic amides, which are solid at room temperature, such as acetamide, are employed as printing inks. U.S. Pat. No. 4,684,956 is directed to monochrome phase change inks utilizing synthetic microcrystalline wax (hydrocarbon wax) and microcrystalline polyethylene wax. This molten composition can be applied to a variety of porous and non-porous substrates using drop-on-demand ink jet application techniques.
Finally, EP Nos. 0187352 and 0206286 reference phase change ink jet printing in color. The ink bases for these systems comprise fatty acids, a thermoplastic polyethylene and a phase change material in the first application; and the alcohol portion of a thermal setting resin pair, a mixture of organic solvents (o-and p-toluene sulfonamide) and a dye in the second application.
There are several references in the prior art to manipulation of images formed from phase change inks, this manipulation taking place either during or after the printing process. In U.S. Pat. No. 4,745,420, droplets of a phase change ink are ejected onto a target and spread thereon by the application of pressure to increase the coverage and minimize the volume of ink that is required to be applied to the surface of the target. In other words, dots of phase change ink which do not initially cover the entire target are spread by pressure application over the entire target surface. Similarly, in xerographic image fusing, the area of contact between the toner and the substrate is substantially increased by causing the toner to spread and penetrate somewhat into the underlying substrate. See "The Physics and Technology of Xerographic Processes", by Williams, published in 1984 by J. Wiley & Sons. The mechanical properties of the toner are such that plastic deformation and flow readily occur. In the case of the phase change ink in U.S. Pat. No. 4,745,420 or the xerographic toner, there is spreading of the ink or toner across the paper to form opaque characters or patterns thereon.
Although the previous references describe fusing of images between a pair of mechanically loaded rollers at ambient temperatures, hot roll fusing has also been used in toner applications. This is a method in which two rolls (one heated) are mechanically loaded together and turned to provide a transient application of heat and pressure to the substrate. The toner is typically heated to above it's glass transition temperature (T.sub.g), which enables it to coalesce, flow, and penetrate the substrate. The rolling pressure and capillary action facilitate coverage. (see "Trends and Advances In Dry Toner Fusing", by Dr. John W. Trainer, Institute For Graphic Communication, June 1985).
Another system for applying phase change inks, U.S. Pat. No. 4,751,528, relates to an ink jet apparatus for the controlled solidification of such inks to assist in controlled penetration of the substrate. The apparatus includes a substrate-supporting, thermally conductive platen and heater and a thermoelectric cooling arrangement, both disposed in heat communication with the platen.
Ink jet printing colored inks on to light transmissive media for displaying color images by overhead projection has historically been a problem. For example, in the case of aqueous inks, special coatings must be provided on the light transmissive medium in order to absorb the solvent so that images of high quality are formed. See U.S. Pat. Nos. 4,503,111, 4,547,405 and 4,555,437. Even though special coatings are not required on receptor films used for phase change ink jet printing, when prior art color phase change inks are printed on transparent substrates the image cannot be used in an overhead projection system. There are two reasons for this. Firstly, the inks are not inherently transparent and do not transmit the majority of the light that impinges on them. The second reason is illustrated in FIG. 1. Drops of phase change ink 12 tend to solidify on the substrate 14 as hemispheres which refract light 16 away from the collection lens 18 of the projection system, in a non-rectilinear path, even if the ink itself is optically transparent. Thus the projected image is visible only in contrast, and most of the colors of the image are not apparent. Therefore to date, phase change inks have not been effectively employed for displaying color images by overhead projection techniques.
Accordingly, a need exists for substrates printed with phase change ink which will transmit light in a rectilinear path so that an image comprising intense colors of a predetermined pattern of such phase change ink can be projected.